Conventionally, a wave plate is manufactured by using an inorganic optical single crystal, such as quartz or the like, and a polymeric oriented film in most cases. However, although the inorganic optical single crystal is superior in performances, durability and reliability for use as a wave plate, its material cost and processing cost are very high. Moreover, the polymeric oriented film tends to easily deteriorate upon application of heat and UV light rays thereto, and has a problem in its durability.
Non-Patent Document 1 has described a wave plate formed by utilizing a structural birefringence, and proposed that by transferring a metal mold having a structural birefringence shape onto a resin material by a nano-printing method, a wave plate is manufactured. More specifically, it is described that by etching a substrate using a dry etching method with a resin material being used as a mask material, the structural birefringence shape can be transferred onto a substrate surface.
In the technique described in this Non-Patent Document 1, however, since a transferring process and an etching process are carried out, complicated processes are required, and it is also necessary to produce a structural body having a very high aspect ratio. For this reason, high etching techniques, such as the use of a resin material that provides a sufficient selection ratio with respect to the substrate material and the improvement of verticality of etching, are required. Consequently, this method has a problem with productivity of a wave plate.
Moreover, Patent Documents 1 to 3 have proposed an optical element that is formed by vapor depositing particles onto a substrate in an oblique direction so as to provide an oblique columnar structure, and allowed to have a birefringence property relative to light rays vertically made incident on a substrate surface. This obliquely vapor deposited film (obliquely vapor deposited wave plate) having the oblique columnar structure, on principle, makes it possible to set an arbitrary phase difference by adjusting the film thickness. Moreover, this method easily provides a device with a large area, and makes is possible to reduce costs by mass production.
In particular, Patent Document 1 has described that an obliquely vapor deposited film, which is composed at least two layers by obliquely vapor depositing a material exerting a high wavelength dispersion and a material exerting a low wavelength dispersion in phase difference, is formed and that this film is allowed to function as a wave plate in a wide band of visible light rays. Further, Patent Document 3 has described that by using an alternate multilayered film between a high-refractive-index medium layer and a low-refractive-index medium layer having a periodic concave/convex shape, a wide operational wavelength can be determined.
However, the above-mentioned conventional obliquely vapor deposited film tends to have a high film thickness so as to achieve a desired birefringence quantity, making it difficult to provide a thin film.